Generally, tetrafluoroethylene (referred to as “TFE”) having high purity has extremely high polymerizability. It is remarkable, when particularly a very small amount of oxygen is contained. It is known to sometimes polymerize explosively.
Thus, when TFE is stored, a polymerization inhibitor, for example, a terpenoid is added in the amount of around 100-10000 ppm (U.S. Pat. No. 2,737,533). There is disclosed a rectification method comprising adding a terpene mixture containing α-pinene, camphene, α-terpinene, D-limonene, γ-terpinene, p-cymene and/or terpinolene so as to prevent the self-polymerization of TFE in a rectifying column simultaneously with lowing the oxygen concentration by absorbing oxygen in TFE, when TFE is rectified (U.S. Pat. No. 3,834,996).
However, the effect of preventing the polymerization of this terpene mixture is not enough in a top of the rectifying column. Polytetrafluoroethylene (hereinafter referred to as “PTFE”) generated by the self-polymerization of TFE accumulates on a tray of the column top, the tray is clogged up with this PTFE so that the operation trouble of the rectifying column is caused. Further, it has been found that the terpene mixture reacts with oxygen and/or acid to give a viscous oligomer compound. When such viscous material has accumulated on the tray of the rectifier, the flooding is caused by a differential pressure increase as by PTFE accumulating. When the flooding occurs, the quality is affected because of poor separation and the throughput should be reduced to have a fear of an influence on productivity.
Disclosed is a method to add α-methyl vinyl benzene and α-methyl vinyl methylbenzene as the inhibitor giving no modified viscous material (U.S. Pat. No. 2,737,533). However, the prevention of polymerization of TFE is said to be effective in a storage step and a transportation step, but enough performance of preventing polymerization can not be obtained in the rectification step.
[Patent document 1] U.S. Pat. No. 2,737,533
[Patent document 2] U.S. Pat. No. 3,834,996